1. Field of the Invention
The present invention relates to extreme ultraviolet lithography (EUVL) systems, and more specifically, it relates to techniques for protecting the optical elements of a EUVL system as well as a wafer in such a system from unwanted out-of-band radiation.
2. Description of Related Art
A spectral purity filter is required in the illuminator of an EUV lithographic system in order to prevent high-power out-of-band radiation from heating up elements in the optical train and to prevent this same radiation from exposing the wafer. The use of an absorbing filter cannot scale to high-power sources due to the thermal load on the foil. It has previously been proposed to use a multilayer-coated blazed diffraction grating to diffract desired EUV light into the optical system (and thereby deflecting the unwanted out-of-band radiation out of the system).
Extreme ultraviolet (EUV) optical systems are being developed for the 11–14 nm wavelength range for applications in semiconductor lithography. An EUV lithographic tool consists of an EUV source, an illuminator optical system (also known as a condenser), a patterned mask, and a projection imaging system. The projection imaging system forms an image of the mask on a resist-coated wafer. The entire optical system consists of reflective surfaces. Most surfaces in the illuminator (and all in the projection optics) are multilayer coated to allow reflection to occur. In order to receive enough power at the wafer to print patterns at a rate that is economically viable, up to 100 Watts of in-band EUV light is required to be collected by the illuminator, due to the fact that multilayer mirrors have reflectivities of less than 72%, and there may be up to 10 or more multilayer surfaces in the system. The EUV source produces a continuous spectrum of light from the infrared to energies higher than the desired EUV light The out-of-band radiation may account for more than 90% of the total integrated power of the source. This power will cause heating of the mirrors, which could degrade their shape, and hence degrade the imaging performance of the system. In addition, the out of band radiation could expose the wafer. Therefore, this power must be prevented from passing through the system.
In the DUV to EUV wavelength range, the multilayer coatings will efficiently absorb all light except that within the bandpass of the coating. This will cause the first multilayer-coated optic in the optical train to heat up, but this power will not be transmitted to subsequent optics. However, visible and IR light is partially reflected by both grazing-incidence and multilayer-coated optics, and so all optics in the optical train will absorb power in this range. In order to prevent distortion of the optical surfaces, substantial cooling is required on the optics, and a spectral purity filter is required that ideally allows only in-band radiation to pass through to subsequent optics.
The spectral purity filter used in experimental EUV lithography systems to date (see, for example, H. N. Chapman et al J. Vac. Sci. Tech. B 19, 2389–2395 (2001).) is a thin foil that is opaque to visible and IR light, and which transmits a broad band in the EUV. Typically these foils are made of silicon, beryllium, zirconium, or other suitable transmitting material. Unfortunately, for these foils to be robust enough to absorb the out of band power, they must be thick (which limits their transmission of in-band EUV to about 50%). The foil can become overheated and damaged. An improved idea (Sweatt, Tichenor and Bernandez, U.S. Pat. No. 6,469,827B1) is to use a grating as a spectral purity filter. The grating is oriented so that the desired in-band EUV light is diffracted into the optical system. Shorter wavelength light will be diffracted by smaller angles and will not be directed onto subsequent optics of the system. Similarly, longer wavelength light will be diffracted by larger angles. Wavelengths, such as IR, greater than the period of the grating will be specularly reflected (in this case the grating acts as a mirror). Specularly reflected light also will not be directed into the optical system, which only receives light that diffracts at a particular range of angles. The grating would be blazed to maximize diffraction efficiency in the desired EUV wavelength. The blaze condition is that the diffracted wavelength reflects specularly from the inclined facets of the grating. The use of a grating has two advantages: the grating can be formed directly on an optical surface of the illuminator, thus avoiding throughput losses from an extra element such as the thin foil; and the out of band power is directed out of the system and is not absorbed in the filter itself as is the case for the foil filter.
The proposal for the grating spectral purity filter of Sweatt, Tichenor and Bernandez is to write a blazed grating onto one of the optical surfaces of the illuminator, which is then overcoated with a multilayer coating. A disadvantage of the proposed technique is that it is difficult to manufacture gratings onto the curved mirror substrates. The most efficient blazed gratings to date have been made by multiple-level electron-beam lithography, which is slow and expensive (Naulleau, U.S. Pat. No. 6,392,792 B1). More conventional holographic and ruled blazed gratings could be manufactured on curved optics, but at significant cost and reduced efficiency. All types of multilayer-coated gratings do suffer inefficiencies due multiple diffracted orders, which are caused by deviations from the ideal saw-tooth shape of the facets and the smoothing action of the multilayer coating. Another disadvantage of the proposed technique is that the optical surface containing the grating has to be redesigned so that specular light is tilted out of the system, and the diffracted light is directed into the system. This requires the element to be tilted, which can lead to aberrations.